A fundamental knowledge of the human immune system is essential if man is to understand the processes involved in immune response, immune deficiency, autoimmune disease and graft rejection. As a contribution to this long range objective, the structure of one protein of the immune system, interleukin 1 beta, will be determined with the method of X-ray crystallography. Interleukin 1 (IL 1) is involved in the regulation of a variety of immunological and inflammatory reactions. IL 1 1) induces interleukin 2 release which in turn stimulates thymocytes to proliferate, 2) stimulates B-lymphocytes to mature and proliferate, 3) exhibits fibroblast growth factor activity, 4) stimulates prostaglandin and collagenase release from synovial cells, and 5) is identical to endogenous pyrogen. The exact role of IL 1 in these processes is still unclear. These uncertainties will be addressed by elucidating the three- dimensional structure of IL 1 beta to high resolution. The structure of IL 1 beta will serve as a basis for a more complete understanding of the function of this molecule. A distantly related protein, interleukin 1 alpha, will be modelled from the structure of IL 1 beta. Since both of these proteins bind to the IL 1 receptor on thymocyte plasma membranes, a comparison of the two structures will help to characterize the regions of the proteins involved in this interaction.